1. Field of the Invention
This invention relates to a key discrimination circuit which determines which key is pushed among keys laid on the crosspoints between two groups of signal lines, particularly, the invention relates to a key discrimination circuit having lower noise generation than that of prior art systems.
2. Description of the Prior Art
According to a conventional key discrimination circuit, as shown in FIG. 1, 9 key switches 7 to 15 are laid on the crosspoints between key input signal lines 1 to 3, which are connected to the 1st key discrimination signal input terminals 24 to 26, and key output signal lines 4 to 6, respectively, each of which is connected to one input terminal of a corresponding one of the 2-input AND gates 16 to 18. The other input terminal of each of the AND gates 16 to 18 is connected to the 2nd key discrimination signal input terminals 19 to 21, respectively. A scanning means 27 is connected to 1st key discrimination signal input terminals 24 to 26 and to 2nd key discrimination signal input terminals 19 to 21.
The outputs of the AND gates are connected to input terminals of a 3-input OR gate 22, the output terminal of which is connected to a key discrimination signal output terminal 23.
An exemplary method to determine a pushed or operated key or key switch in this conventional key discrimination circuit is described below. A cycle is repeated in which the 2nd key discrimination signal input terminals 19 to 21 have a "1" level pulse signal cyclically applied thereto from scanning means 27 when only line 1 among the key input signal lines 1 to 3 has a "1" level signal applied thereto from scanning means 27 and the other key input signal lines are held at a "0" level; then, the key discrimination signal input terminals 19 to 21 again have a "1" level pulse signal cyclically applied thereto from scanning means 27 when only line 2 among key input signal lines 1 to 3 has a "1" level signal applied thereto from scanning means 27 and the other key input signal lines are held at a "0" level; and lastly, the key discrimination signal input terminals 19 to 21 have a "1" level pulse signal cyclically applied thereto from scanning means 27 when only line 3 among key signal lines 1 to 3 has a "1" level signal applied thereto from scanning means 27 and the other key input signal lines are held at a "0" level. In such a procedure, the key among key switches 7 to 15 which is pushed is determined by judging which line among input signal lines 1 to 3 is held at a "1" level and which terminal among key discrimination signal input terminals 19 to 21 has a "1" level pulse signal applied thereto from scanning means 27 at that time. That is, by the simultaneous or synchronous supply of "1" level signals to a key input signal line and an input terminal of an AND gate by scanning means, the key switch corresponding to both is determined as operated key switch.
However, in such a conventional key discrimination circuit, key switches 7 to 15 laid on the crossing points among key input signal lines 1 to 3 and key output signal lines 4 to 6 and the circuit block containing the 1st key discrimination signal input terminals 24 to 26 and the 2-input AND gates 16 to 18 which are connected to the key output signal lines 4 to 6 are separated, so that the key input signal lines 1 to 3 and key output signal lines 4 to 6 are actually long lines connected between the keys and the circuit block. Key input signal lines 1 to 3 often generate noise to other circuit blocks when these key input signal lines 1 to 3 are driven by a pulse signal from scanning means 27, the pulse signal having a "1" level and a "0" level in turn. For example, if an FM/AM radio broadcasting receiver is set adjacent to this key discrimination circuit, the reception of FM/AM radio broadcasting receiver is disturbed by this noise.